The present invention relates to a linear motor having a structure in which permanent magnets and a coil are subjected to relative movement, and an apparatus and a method for protecting such a linear motor.
Movable coil-type linear motors have conventionally been widely used as driving means for positioning articles in stroke ranges of about 10-100 cm (for instance, Japanese Patent Publication No. 58-49100 and Japanese Utility Model Laid-Open No. 63-93783). The movable coil-type linear motor comprises a plurality of permanent magnets magnetized in their thickness directions and arranged such that their different magnetic poles are opposing each other, and a movable coil assembly moving in a magnetic gap defined between the opposing permanent magnets (or between permanent magnets and a yoke) in perpendicular to a magnetic flux.
Such a linear motor is free from a center yoke in a magnetic circuit portion, and comprises a plurality of closed-loop magnetic fluxes in the magnetic gap, so that a magnetic flux is not concentrated in part of the magnetic path. Accordingly, this linear motor can generate a uniform magnetic flux density in an overall range of a long stroke.
FIG. 5 schematically shows one example of a mechanical portion of the linear motor, in which permanent magnets and a coil are subjected to relative movement. This linear motor comprises a pair of flat plate-shaped yokes 1, 1 made of ferromagnetic materials such as soft iron, a pair of permanent magnet rows 102, 102 constituted by a plurality of permanent magnets 2 magnetized in their thickness directions, and attached to the inner surface of the flat plate-shaped yokes 1, 1 respectively, such that they are opposing via a magnetic gap 3, and supports 4, 4 attached to both ends of a pair of yokes 1, 1 to provide the magnetic gap 3. In each permanent magnet row 102, the permanent magnets 2 are arranged on each yoke 1, 1 in a longitudinal direction, such that N poles and S poles alternately appear on the surfaces of the permanent magnets 2, and that different magnetic poles appear on the surfaces of the permanent magnets 2 opposing via the magnetic gap 3. A plurality of closed-loop magnetic circuits are constituted by a pair of yokes 1, 1 and a pair of opposing permanent magnets 2, 2 (refer to FIG. 5). Incidentally, the supports 4 are preferably formed by the same ferromagnetic materials as those of the yokes 1.
FIG. 6 schematically shows another example of the linear motor. This linear motor comprises a plurality of permanent magnets 2 arranged on one yoke 1 to constitute magnetic circuits, instead of having a structure that the different poles of a plurality of permanent magnets 2 are opposing via the magnetic gap 3.
FIGS. 5 and 6 respectively show movable coil-type, linear motors, both of which are essentially the same except for difference in that the number of the combination of a permanent magnet row 102 and a yoke 1 is two or one. Because the linear motor of FIG. 5 has a larger magnetic flux density at the same current level, it provides a larger thrust.
The coil 5 is constituted by flat multi-phase coils with the winding direction of the coil 5 in perpendicular to a magnetic flux direction in a magnetic gap 3. A plurality of coils 5 (only one coil is shown for simplicity) are arranged longitudinally along the permanent magnet row 102, and the directions of their magnetic poles are detected to switch a coil to which current is supplied and the direction of current by a means such as a magnetic field detecting element, etc.
In the movable coil-type linear motor, a plurality of coils 5 are arranged in a stroke direction to generate a large thrust. A plurality of coils 5 are integrally fixed to a non-magnetic holder (not shown) to constitute a mover. The mover is movably supported by a sliding member (not shown) in a longitudinal direction of a permanent magnet row 102, and the holder is integrally fixed to a table (carriage) on which an article is placed. The holder is made of non-magnetic materials such as resins, aluminum, ceramics, etc. so as not to provide magnetic influence on a closed-loop magnetic circuit. To make the magnetic gap 3 as small as possible, the holder is preferably as thin as possible.
In order that the coil 5 receives thrust to move in a constant direction, the direction of current should be changed successively according to the polarity of the opposing permanent magnets 2. When the coil 5 faces a boundary of the adjacent permanent magnets 2, 2, there is no thrust of movement. Current supply is thus stopped to the coil 5. The holder is equipped with one element (usually Hall element) for detecting the polarity of the permanent magnets 2 per one phase of the coil. Accordingly, in the case of the three-phase coil, the holder is equipped with three detecting elements.
An arranging pitch is not the same for the coil 5 and the permanent magnets 2. If both were the same, there would appear a moment in which the combined force of the thrust of each coil 5 is zero, resulting in a large thrust ripple, which leads to the cogging of the coil 5. With the arranging pitch of the coil 5 and the permanent magnets 2 deviated form each other, the coil 5 undergoes smooth movement with a reduced thrust ripple. In that case, electric current should be supplied successively to part of a plurality of coils 5. The timing of ON/OFF to supply electric current to each coil 5 and the direction of electric current are determined depending on the output of the hole element.
When current is supplied to the coil 5, it receives thrust in the longitudinal direction of the yoke 1 by the Fleming""s rule, a mover (not shown) integrally provided with the coil 5 moves in the longitudinal direction of the yoke 1. When current is supplied to the coil 5 in an opposite direction, thrust in an opposite direction acts on coil 5, resulting in moving the mover in an opposite direction. Accordingly, current supply to the coil 5 and the direction of that current can be chosen to move the mover to the predetermined position. The strength of this thrust is proportional to current flowing through the coil 5.
Contrary to the above embodiment, the coil may be a stator, and the permanent magnets may be movers to achieve the same function. Though the coil 5 is disposed in the magnetic gap 3 in the above embodiment, as shown in FIG. 6, a linear motor may be free from a magnetic gap 3. In such a linear motor, the coil 5 is movable on permanent magnets 2 disposed on a yoke 1.
When current is caused to flow through the coil 5, Joule heat is generated, and when the temperature of the coil 5 is elevated to a level exceeding its heat resistance limit, the coil 5 is burned down. As a method preventing this problem, for instance, Japanese Patent Laid-Open No. 4-67763 proposes the mounting of a temperature-detecting element such as a thermistor, etc. to a coil. However, though a surface portion of the coil is well cooled by air-flowing effect, etc., heat is likely to be accumulated inside the coil, resulting in the tendency that the temperature of the coil is higher in an inner portion than in a surface portion. Because a temperature-detecting element such as a thermistor, etc. is mounted onto the coil surface, it is impossible to detect the temperature inside the coil. Accordingly, it has been found that the method disclosed in Japanese Patent Laid-Open No. 4-67763 fails to accurately detect the temperature at which the coil 5 is burned down. In addition, it suffers from the problem that the mounting of a thermistor, etc. onto the coil surface makes wiring, etc. more difficult.
To avoid such problems, there is a known method for generating a coil overheat signal when the product of Ixc3x97T, in which I is a level of current flowing through a coil, and T is time in which the current flows, exceeds a predetermined value. As one example thereof, Japanese Patent Laid-Open No. 59-89517 proposes an electronic overcurrent relay for generating an overload signal for a time period of operation corresponding to the current level, when alternating current in a main apparatus exceeds a predetermined value. Japanese Patent Laid-Open No. 63-209420 proposes an electronic thermal relay having operation characteristics obtained by improving the electronic overcurrent relay of Japanese Patent Laid-Open No. 59-89517 in a range of a large current.
In general, an induction motor achieves the maximum speed in a steady operation, in which it is required to provide the maximum torque. Accordingly, it is necessary to detect overload to load variations in this state. Effective as an overload-detecting apparatus is an electronic overcurrent relay or an electronic thermal relay, with which Ixc3x97T is calculated.
On the other hand, the maximum speed is achieved in a linear motor in a steady operation, and the speed can be kept with a relatively small current necessary for compensating deceleration by a running resistance in this state. The maximum thrust is required rather at the time of acceleration or deceleration, causing large current to flow through the linear motor. Accordingly, it is necessary to accurately detect the overheat of the coil by current measurement for a short period of time, during acceleration or deceleration at which large current flows for a short period of time, rather than in a steady operation in which the maximum speed is achieved.
Used in the control of a linear motor are a position deviation signal obtained by subtracting the feedback value of position information from the predetermined value of position information, and a speed deviation signal obtained by subtracting the feedback value of speed information from the predetermined value of speed information. In general, current in proportion to the position deviation signal and the speed deviation signal is supplied to a coil in a linear motor in a steady operation. However, when a mover has fallen into a full stop by accident in the course of movement, or when a mover has stopped, for instance, before reaching a predetermined stop position because of overload (particularly when it has stopped immediately before reaching the predetermined stop position), the level of current supplied to the coil is kept relatively low because of a small position deviation signal (position deviation), failing to restart the mover. With this current supply condition kept for a long period of time, a Joule heat is accumulated in the coil. If current supply were not stopped soon in this state, the coil would be overheated.
Though the conventional electronic overcurrent relay or electronic thermal relay have good operation characteristics in a range of a large current, they cannot properly detect the overheat of the coil when a relatively small current flows for a long period of time. If the predetermined value of overload-detecting current were made smaller to detect the overheat of a coil, even small current after reaching the predetermined speed would lead to the determination that the coil is overheated, resulting in trouble in the operation of the linear motor.
In the case of the linear motor comprising a multi-phase coil, as described above, a plurality of coils are arranged with deviations corresponding to an exciting period in the arrangement direction of the permanent magnets. For instance, in the case of a three-phase coil having at least three coils arranged in a stroke direction, the pitch of the coils is 120xc2x0 relative to a period of a magnetic field (one magnetic field period with two magnets). The direction of a magnetic pole is detected by a means such as a magnetic field-detecting element, etc. To make it possible to switch coils through which current should flow and its direction, it is preferable to control the driving of the multi-phase coils independently.
However, in the conventional apparatus for protecting a linear motor, which has wiring via Y-connection or xcex94-connection, for instance, in the case of three-phase coils, current is detected only in one or two phases among the three phases to mainly control the level of current. The detected level of current is also used as operation information for a protecting means. Therefore, a current level (effective value) of each phase differs depending on the deviations of a magnetic field, the impedance of each coil, etc. In some cases, an undetected phase may have a high current level, while a detected phase may have a low current level. In this case, excess current in the undetected phase leads to the overheat of the coil.
Accordingly, an object of the present invention is to provide an apparatus and a method for protecting a linear motor, which can accurately detect the overheat of a coil by the measurement of current in a short period of time during acceleration or deceleration in which large current flows in a short period of time, which do not detect the overheat of a coil in a state where small current flows after a mover has reached a predetermined speed, and which can detect the overheat of a coil when the mover has stopped in the course of movement, resulting in relatively small current flowing for a long period of time, whereby Joule heat is accumulated, and a linear motor equipped with such a protecting apparatus.
Another object of the present invention is to provide an apparatus and a method for protecting a linear motor comprising a multi-phase coil, which can detect current in all phases to prevent the overheat, and a linear motor comprising a multi-phase coil equipped with such a protecting apparatus.
The apparatus of the present invention for protecting a linear motor having a structure in which permanent magnets and a coil are subjected to relative movement, comprises (a) a means for detecting current flowing through said coil, (b) a multiplying means for calculating the squared value of the detected current every constant time xcex94t, (c) a memory means receiving said squared values for successively storing said squared values from the first one to the nth one, the first squared value being eliminated when the (n+1)th squared value is input, and the same operation being repeated subsequently, (d) an adding means for calculating the total of n squared values in said memory means, (e) a means for setting a reference value, which is to be compared with the total of said squared values, (f) a means for comparing the total of said squared values with said reference value for outputting an overload signal when said total has become larger than said reference value, and (g) an overload-preventing means receiving said overload signal for controlling a driving means of said coil, thereby protecting said coil.
The method of the present invention for protecting a linear motor having a structure in which permanent magnets and a coil are subjected to relative movement, comprises the steps of (1) detecting current flowing through said coil, (2) calculating the squared value of the detected current every constant time xcex94t, (3) successively storing said squared values from the first one to the nth one, eliminating the first squared value when the (n+1)th squared value is input, and repeating the same operation subsequently, (4) calculating the total of n squared values, (5) comparing the total of said squared values with a reference value, (6) outputting an overload signal when said total has become larger than said reference value, and (7) controlling a driving means of said coil when said overload signal is received, thereby protecting said coil.
The linear motor of the present invention has a structure in which permanent magnets and a coil are subjected to relative movement, comprising a protecting apparatus comprising (a) a means for detecting current flowing through said coil, (b) a multiplying means for calculating the squared value of the detected current every constant time xcex94t, (c) a memory means receiving said squared values for successively storing said squared values from the first one to the nth one, the first squared value being eliminated when the (n+1)th squared value is input, and the same operation being repeated subsequently, (d) an adding means for calculating the total of n squared values in said memory means, (e) a means for setting a reference value, which is to be compared with the total of said squared values, (f) a means for comparing the total of said squared values with said reference value for outputting an overload signal when said total has become larger than said reference value, (g) an overload-preventing means receiving said overload signal for controlling a driving means of said coil, thereby protecting said coil.
The apparatus for protecting a linear motor having a structure in which permanent magnets and a multi-phase coil are subjected to relative movement, comprises in each phase of said multi-phase coil, (a) a means for detecting current flowing through said coil, (b) a multiplying means for calculating the squared value of the detected current every constant time xcex94t, (c) a memory means receiving said squared values for successively storing said squared values from the first one to the nth one, the first squared value being eliminated when the (n+1)th squared value is input, and the same operation being repeated subsequently, (d) an adding means for calculating the total of n squared values in said memory means, (e) a means for setting a reference value, which is to be compared with the total of said squared values, (f) a means for comparing the total of said squared values with said reference value for outputting an overload signal when said total has become larger than said reference value, and (g) an overload-preventing means receiving said overload signal for controlling a driving means, thereby protecting said multi-phase coil.
The method of the present invention for protecting a linear motor having a structure in which permanent magnets and a multi-phase coil are subjected to relative movement, comprises in each phase of said multi-phase coil the steps of (1) detecting current flowing through said coil, (2) calculating the squared value of the detected current every constant time xcex94t, (3) inputting said squared values to successively store said squared values from the first one to the nth one, eliminating the first squared value when the (n+1)th squared value is input, and repeating the same operation subsequently, (4) calculating the total of n squared values, (5) comparing the total of said squared values with a reference value, (6) outputting an overload signal when said total has become larger than said reference value, and (7) controlling a driving means of said coil when said overload signal is received, thereby protecting said multi-phase coil.
The linear motor of the present invention has a structure in which permanent magnets and a multi-phase coil are subjected to relative movement, comprising in each phase of said multi-phase coil a protecting means comprising (a) a means for detecting current flowing through said coil, (b) a multiplying means for calculating the squared value of the detected current every constant time xcex94t, (c) a memory means receiving said squared values for successively storing said squared values from the first one to the nth one, the first squared value being eliminated when the (n+1)th squared value is input, and the same operation being repeated subsequently, (d) an adding means for calculating the total of n squared values in said memory means, (e) a means for setting a reference value, which is to be compared with the total of said squared values, (f) a means for comparing the total of said squared values with said reference value for outputting an overload signal when said total has become larger than said reference value, (g) an overload-preventing means receiving said overload signal for controlling a driving means, thereby protecting said multi-phase coil.